Formation and Analysis of Biofilms in Vivo.

Establishing a biofilm infection model in vivo allows a better understanding of the underlying infection mechanisms of bacteria. Here we describe a method for constructing an in vivo biofilm model of Streptococcus suis. The animal modeled is a piglet, which is the natural reservoir of S. suis, and the mode of clinical infection is simulated by intranasal inoculation of S. suis. This model is in line with clinical practice, easy to operate, and has good repeated stability.

AI-2 quorum sensing-induced galactose metabolism activation in Streptococcus suis enhances capsular polysaccharide-associated virulence.

Bacteria utilize intercellular communication to orchestrate essential cellular processes, adapt to environmental changes, develop antibiotic tolerance, and enhance virulence. This communication, known as quorum sensing (QS), is mediated by the exchange of small signalling molecules called autoinducers. AI-2 QS, regulated by the metabolic enzyme LuxS (S-ribosylhomocysteine lyase), acts as a universal intercellular communication mechanism across gram-positive and gram-negative bacteria and is crucial for diverse bacterial processes. In this study, we demonstrated that in Streptococcus suis (S. suis), a notable zoonotic pathogen, AI-2 QS enhances galactose utilization, upregulates the Leloir pathway for capsular polysaccharide (CPS) precursor production, and boosts CPS synthesis, leading to increased resistance to macrophage phagocytosis. Additionally, our molecular docking and dynamics simulations suggest that, similar to S. pneumoniae, FruA, a fructose-specific phosphoenolpyruvate phosphotransferase system prevalent in gram-positive pathogens, may also function as an AI-2 membrane surface receptor in S. suis. In conclusion, our study demonstrated the significance of AI-2 in the synthesis of galactose metabolism-dependent CPS in S. suis. Additionally, we conducted a preliminary analysis of the potential role of FruA as a membrane surface receptor for S. suis AI-2.

Streptococcal arginine deiminase system defences macrophage bactericidal effect mediated by XRE family protein XtrSs.

The arginine deiminase system (ADS) has been identified in various bacteria and functions to supplement energy production and enhance biological adaptability. The current understanding of the regulatory mechanism of ADS and its effect on bacterial pathogenesis is still limited. Here, we found that the XRE family transcriptional regulator XtrSs negatively affected Streptococcus suis virulence and significantly repressed ADS transcription when the bacteria were incubated in blood. Electrophoretic mobility shift (EMSA) and lacZ fusion assays further showed that XtrSs directly bind to the promoter of ArgR, an acknowledged positive regulator of bacterial ADS, to repress ArgR transcription. Moreover, we provided compelling evidence that S. suis could utilize arginine via ADS to adapt to acid stress, while ΔxtrSs enhanced this acid resistance by upregulating the ADS operon. Moreover, whole ADS-knockout S. suis increased arginine and antimicrobial NO in the infected macrophage cells, decreased intracellular survival, and even caused significant attenuation of bacterial virulence in a mouse infection model, while ΔxtrSs consistently presented the opposite results. Our experiments identified a novel ADS regulatory mechanism in S. suis, whereby XtrSs regulated ADS to modulate NO content in macrophages, promoting S. suis intracellular survival. Meanwhile, our findings provide a new perspective on how Streptococci evade the host's innate immune system.

Fpr2 exacerbates Streptococcus suis-induced streptococcal toxic shock-like syndrome via attenuation of neutrophil recruitment.

The life-threatening disease streptococcal toxic shock-like syndrome (STSLS), caused by the bacterial pathogen Streptococcus suis (S. suis). Proinflammatory markers, bacterial load, granulocyte recruitment, and neutrophil extracellular traps (NETs) levels were monitored in wild-type (WT) and Fpr2-/- mice suffering from STSLS. LXA4 and AnxA1, anti-inflammatory mediators related to Fpr2, were used to identity a potential role of the Fpr2 in STSLS development. We also elucidated the function of Fpr2 at different infection sites by comparing the STSLS model with the S. suis-meningitis model. Compared with the WT mice, Fpr2-/- mice exhibited a reduced inflammatory response and bacterial load, and increased neutrophil recruitment. Pretreatment with AnxA1 or LXA4 impaired leukocyte recruitment and increased both bacterial load and inflammatory reactions in WT but not Fpr2-/- mice experiencing STSLS. These results indicated that Fpr2 impairs neutrophil recruitment during STSLS, and this impairment is enhanced by AnxA1 or LXA4. By comparing the functions of Fpr2 in different S. suis infection models, inflammation and NETs was found to hinder bacterial clearance in S. suis meningitis, and conversely accelerate bacterial clearance in STSLS. Therefore, interference with neutrophil recruitment could potentially be harnessed to develop new treatments for this infectious disease.

Interaction between gut microbiota dysbiosis and lung infection as gut-lung axis caused by Streptococcus suis in mouse model.

Streptococcus suis (S. suis) is an important zoonotic pathogen threatening the global pig farming industry. It causes respiratory and digestive tract infections simultaneously in pigs. The balanced gut microbiota not only affects the local mucosal immune response but also involves the regulation of the immune status of the distant lung tissues that is termed as "gut-lung" axis. Whether S. suis affects the gut during lung infection and how does the intestinal microbial disturbance play role in the development of lung infection during S. suis exposure is not clear yet. Therefore, in the current study, we constructed the animal model using six-week-old mice (N = 48) divided into four groups with S. suis serotype 2 (SS2)-induced lung infection and the antibiotic treated gut microbiota dysbiosis. By means of various techniques (like HE staining, RT-qPCR, Western Blot and ELISA and viability detection) we explored that S. suis can concurrently cause intestinal tissue damage and inflammation after lung infection. Moreover, gut microbiota dysbiosis changes the balance of Th1/Th2 cells that aggravates lung injury during the infection. Thus, "gut-lung" axis of the communication between the gut microbiota and lung infection was established through the spleen and blood. In addition, intestinal dysbacteriosis can affect alveolar macrophage activity for a long time and the balance of gut microbiota plays an important role in lung infection caused by S. suis. Hence, this study clarified the pulmonary infection caused by SS2 from the perspective of the intestinal microbiota providing novel theoretical basis for the treatment of related lung diseases.

HP0487 contributes to the virulence of Streptococcus suis serotype 2 by mediating bacterial adhesion and anti-phagocytosis to neutrophils.

Streptococcus suis serotype 2 (SS2) is an important zoonotic pathogen that poses a serious threat to human health and the swine industry. The survival and travel in the bloodstream are the important causes for SS2, contributing to bacteremia, septicemia even septic shock. However, the related mechanism remains largely unknown. Preliminary experiment demonstrated that SS2 could largely attach to the surface of neutrophils, implying that this phenomenon maybe contributed to the travel of SS2 in bloodstream and then influenced its pathogenicity. To confirm this hypothesis, using a previously established screening method that combines affinity chromatography (based on liquid chromatography-tandem mass spectrometry) with shotgun proteomics, three candidate proteins (HP0487, HP1765, and HP1111) were identified from SS2 that could interact with neutrophils. Next, by constructing the deletion mutations, we demonstrated that HP0487 of three proteins could significantly influence the adhesion of SS2 to neutrophils. Furthermore, HP0487 was shown to contribute to the anti-phagocytosis of SS2 to neutrophils and RAW264.7 cells. More importantly, the deletion of HP0487 significantly reduced lethality and bacterial loads in vivo of SS2. Thus, our findings demonstrate that HP0487 contributes to SS2 virulence by mediating the adhesion and anti-phagocytosis of SS2 to neutrophils, promoting a better understanding about the pathogenesis of SS2.

Genome-wide association study identifies the virulence-associated marker in Streptococcus suis serotype 2.

Streptococcus suis (S. suis) has been reported to be a highly invasive pathogen in swine, which causes severe infections like meningitis, arthritis and septicemia, and also a zoonotic agent for humans. Although many putative virulence factors (VFs) have been identified, the exact and wildly accepted virulence associated marker and pathogenesis mechanism of S. suis are still unclear. To establish connection of the genotypes with virulence phenotypes, we performed an "internal standard" method based on the zebrafish model to assess the virulence phenotypes of S. suis and did the genome-wide association study (GWAS) based on the genomes of 68 S. suis isolates. Through GWAS, a total number of 172 genes were identified. Among these genes, 143 of them distribute in virulent isolates. Further VFs interaction network analysis based on protein-protein interaction database found that 71 genes identified in this study could interact with known VFs and some of them even played an important role as the bridge between known VFs or formed important hub. In addition, 12 genes were found conserved in virulent isolates and 3 genes were conserved in avirulent isolates, 8 genes of the virulent conserved genes were belonging to a srtBCD pili cluster. Considering that sbp2', a member of the srtBCD pili cluster has been reported as a virulence-associated factor, we predict that sbp2' could be a fitness virulence-associated marker of virulent isolates. Taken together, our findings contribute to the insights in S. suis pathogenesis, enhance the knowledge of the genomic evolution of S. suis and provide several novel virulence-associated candidates.

Review of the speculative role of co-infections in Streptococcus suis-associated diseases in pigs.

Streptococcus suis is one of the most important bacterial swine pathogens affecting post-weaned piglets, causing mainly meningitis, arthritis and sudden death. It not only results in severe economic losses but also raises concerns over animal welfare and antimicrobial resistance and remains an important zoonotic agent in some countries. The definition and diagnosis of S. suis-associated diseases can be complex. Should S. suis be considered a primary or secondary pathogen? The situation is further complicated when referring to respiratory disease, since the pathogen has historically been considered as a secondary pathogen within the porcine respiratory disease complex (PRDC). Is S. suis a respiratory or strictly systemic pathogen? S. suis is a normal inhabitant of the upper respiratory tract, and the presence of potentially virulent strains alone does not guarantee the appearance of clinical signs. Within this unclear context, it has been largely proposed that co-infection with some viral and bacterial pathogens can significantly influence the severity of S. suis-associated diseases and may be the key to understanding how the infection behaves in the field. In this review, we critically addressed studies reporting an epidemiological link (mixed infections or presence of more than one pathogen at the same time), as well as in vitro and in vivo studies of co-infection of S. suis with other pathogens and discussed their limitations and possibilities for improvement and proposed recommendations for future studies.

The otc gene of Streptococcus suis plays an important role in biofilm formation, adhesion, and virulence in a murine model.

Streptococcus suis (S. suis) is an emerging zoonotic pathogen that can cause meningitis, arthritis, pneumonia, and sepsis. It poses a serious threat to the swine industry and public health worldwide. Ornithine carbamoyltransferase (OTC) is involved in the arginine deiminase system. OTC, which is a widely distributed enzyme in microorganisms, mammals, and higher plants, catalyzes the conversion of ornithine to citrulline. The present study showed that the otc gene plays an important role in the pathogenesis of S. suis infections. The ability of an otc-deficient mutant (Δotc) to form a biofilm was significantly reduced compared to the wild-type (WT) strain, as determined by crystal violet staining. Confocal laser scanning microscopy and scanning electron microscopy observations showed that the weakening of biofilm formation by the Δotc strain is related to a decrease in the extracellular matrix. In addition, compared to the WT strain, the Δotc strain had a reduced capacity to adhere to human laryngeal epidermoid carcinoma (HEp-2) cells compared to the WT strain. A real-time PCR analysis showed that the expression of adhesion-related genes by the Δotc strain was also lower than that of the WT strain. The virulence of the Δotc strain was significantly lower than that of the WT strain in a murine infection model. In addition, a histological analysis showed that the pathogenicity of the Δotc strain was lower than that of the WT strain, causing only slight inflammatory lesions in lung, liver, spleen, and kidney tissues. No significant differences were observed between the complemented mutant (CΔotc) and WT strains with respect to biofilm formation, adhesion, gene expression, and virulence. The present study provided evidence that the otc gene plays a pivotal role in the regulation of S. suis adhesion and biofilm formation. It also suggested that the otc gene is indirectly involved in the pathogenesis of S. suis serotype 2 infections.

Identification of a Novel Linear B Cell Epitope on the Sao Protein of Streptococcus suis Serotype 2.

Surface antigen one (Sao) protein is a bacterial surface protein identified in the important zoonotic pathogen Streptococcus suis serotype 2 (S. suis 2) during an extensive search for functional proteins. The Sao protein is anchored to the bacterial cell wall by the LPVTG motif and is widely distributed in many S. suis serotypes. In this paper, we present the immunodominant epitope peptide of the Sao protein that is recognized by BALB/c antibodies against the Sao protein: 355SEKQMPSVVNENAVTPEKQMTNKENDNIET384 (location Sao355-384). To determine the core epitope recognized by antibodies, we prepared truncation peptide libraries. Analyses of the immunoreactivity of truncation peptides with anti-Sao355-384 serum revealed that the most immunoreactive sequence was 355SEKQMPSVVNENAVTPEK372 (location Sao355-372). Moreover, we observed that this core epitope also showed good specificity based on the ratio of reactivity with serum from S. suis-positive patients compared to serum from S. suis-negative patients. Our results point to the potential of using the Sao355-372 peptide in diagnostic assays to determine S. suis infection in humans.

Population structure, genetic diversity and pathotypes of Streptococcus suis isolated during the last 13 years from diseased pigs in Switzerland.

Streptococcus (S.) suis is a globally important swine pathogen, which comprises certain zoonotic serotypes. In this study, a detailed characterization of 88 porcine S. suis isolates was performed by analyzing capsular (cps) types, multilocus sequence typing (MLST) and investigation of the minimum core genome (MCG). In order to focus on the virulence potential of presumable invasive disease-associated S. suis isolates, virulence-associated gene profiles were assessed followed by screening a chosen subset of S. suis strains with a molecular pathotyping tool. Results showed a high genetic variability within this strain collection. In total, seventeen cps types were identified with a predominance of cps type 9 (15.9%) and 6 (14.8%). MLST revealed 48 sequence types (STs) including 41 novel ones. The population structure of S. suis was heterogenous and isolates belonged to eight different clonal complexes (CCs) including CC28 (9.1%), CC1109 (8%), CC13/149 (6.8%), CC1237 (5.7%), CC1 (3.4%), CC17 (3.4%), CC87 (2.3%), and CC1112 (1.1%), whereas a significant portion of isolates (60.2%) could not be assigned to any described CCs. Virulence-associated markers, namely extracellular protein factor (epf), muramidase-released protein (mrp), and suilysin (sly), showed a link with STs rather than with cps types. With this study an expanded knowledge about the population structure and the genetic diversity of S. suis could be achieved, which helps to contribute to an optimal public health surveillance system by promoting a focus on strains with an increased virulence and zoonotic potential.

Early infection of Streptococcus suis serotype 2 increases the virulence of highly pathogenic porcine reproductive and respiratory syndrome MLV-like virus in pigs.

Modified-live virus (MLV) vaccines derived from highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) were wildly used in China, which resulted in the emergence of MLV-like strains in pigs. Previous studies demonstrated that secondary bacterial infection could enhance HP-PRRSV infection-mediated inflammatory responses, but it is unknown whether early bacterial infection could enhance the HP-PRRSV MLV-like infection-mediated pathological reaction. In this paper, to gain the evidence for infection of pigs with MLV-like strains in China, we firstly analyzed the genetic characterization of the HP-PRRSV MLV-like isolate (TJxq1701) and further evaluated whether the early Streptococcus suis infection synergizes HP-PRRSV MLV-like infection-mediated pathological reaction. Our results showed that the whole genome of TJxq1701 shared the highest homology with JXA1-P80 and a total of 16 amino acids residues unique to JXA1-P80 in ORF1a, ORF1b, GP2, GP3, GP4, and GP5 were found in the corresponding locations. The results of infection experiments in pigs revealed that TJxq1701 caused transitional fever, moderate respiratory clinical sign and microscopic lung lesions in piglets, but early infection with low virulence Streptococcus suis serotype 2 (SS2) exhibited seriously clinical signs, including high fever, anorexia, and respiratory distress, leading to 60% mortality within four weeks in comparison with alone infected group. Taken together, our findings reveal that early bacterial infection could enhance the HP-PRRSV MLV-like infection-mediated pathological reaction, which provide an important clue for understanding that streptococcus infection increases the pathogenicity of MLV-like virus and a new thought for prevention and control of PRRSV.

Porcine circovirus type 2 exploits JNK-mediated disruption of tight junctions to facilitate Streptococcus suis translocation across the tracheal epithelium.

Porcine circovirus type 2 (PCV2) is considered as the primary pathogen of porcine circovirus-associated disease (PCVAD), which results in significant economic losses worldwide. Clinically, PCV2 often causes disease through coinfection with other bacterial pathogens, including Streptococcus suis (S. suis), and especially the highly prevalent S. suis serotype 2 (SS2). The present study determined that continuous PCV2 infection in piglets down-regulates tight junction proteins (TJ) ZO-1 and occludin in the lungs. Swine tracheal epithelial cells (STEC) were used to explore the mechanisms and consequences of disruption of TJ, and an in vitro tracheal epithelial barrier model was established. Our results show that PCV2 infection in STEC decreases the expression levels of ZO-1 and occludin and increases the permeability of the tracheal epithelial barrier, resulting in easier translocation of SS2. Moreover, Western blot analysis indicates that PCV2 infection activates the JNK/MAPK pathway. The disruption of TJ in SETC and increased permeability of the epithelial barrier induced by PCV2 could be alleviated by inhibition of JNK phosphorylation, which indicates that the JNK/MAPK pathway regulates the expression of ZO-1 and occludin during PCV2 infection. This study allows us to better understand the mechanisms of PCV2 coinfection with bacterial pathogens and provides new insight into controlling the occurrence of PCVAD.

Pdh is involved in the cell division and Normal septation of Streptococcus suis.

Streptococcus suis (S. suis) is an important zoonotic pathogen that causes major economic losses in the pig industry worldwide. The S. suis cell division process is an integral part of its growth and reproduction, which is controlled by a complex regulatory network. Pyruvate dehydrogenase (PDH), which catalyzes the oxidative decarboxylation of pyruvate to form acetyl-CoA, while reducing NAD + to NADH, plays an important role in energy metabolism. Recently, we reported that pdh regulates virulence by reducing stress tolerance and biofilm formation in S. suis serotype 2. In this study, we found that deletion of the pdh gene in S. suis resulted in abnormal cell chains, plump morphology and abnormal localization of the Z rings, indicating that the knockout mutant is impaired in its ability to divide. In addition, the interaction between FtsZ and PDH in vitro was confirmed by ELISA, and qRT-PCR analysis revealed that the deletion of the pdh gene results in differential expression of the division-related genes ftsZ, ftsK, ftsl, zapA, divIC, pbp1a, rodA, mreD, and sepF. These results indicate that pdh is involved in the normal formation of Z rings and cell morphology during S. suis cell division.

Interleukin-1 signaling induced by Streptococcus suis serotype 2 is strain-dependent and contributes to bacterial clearance and inflammation during systemic disease in a mouse model of infection.

Streptococcus suis serotype 2 is an important porcine pathogen and zoonotic agent causing sudden death, septic shock and meningitis, with exacerbated inflammation being a hallmark of the infection. A rapid, effective and balanced innate immune response against S. suis is critical to control bacterial growth without causing excessive inflammation. Even though interleukin (IL)-1 is one of the most potent and earliest pro-inflammatory mediators produced, its role in the S. suis pathogenesis has not been studied. We demonstrated that a classical virulent European sequence type (ST) 1 strain and the highly virulent ST7 strain induce important levels of IL-1 in systemic organs. Moreover, bone marrow-derived dendritic cells and macrophages contribute to its production, with the ST7 strain inducing higher levels. To better understand the underlying mechanisms involved, different cellular pathways were studied. Independently of the strain, IL-1ß production required MyD88 and involved recognition via TLR2 and possibly TLR7 and TLR9. This suggests that the recognized bacterial components are similar and conserved between strains. However, very high levels of the pore-forming toxin suilysin, produced only by the ST7 strain, are required for efficient maturation of pro-IL-1ß via activation of different inflammasomes resulting from pore formation and ion efflux. Using IL-1R-/- mice, we demonstrated that IL-1 signaling plays a beneficial role during S. suis systemic infection by modulating the inflammation required to control and clear bacterial burden, thus promoting host survival. Beyond a certain threshold, however, S. suis-induced inflammation cannot be counterbalanced by this signaling, making it difficult to discriminate its role.

Antibacterial activity against porcine respiratory bacterial pathogens and in vitro biocompatibility of essential oils.

Bacterial respiratory infections affecting pigs such as pneumonia, pleuropneumonia, and pleurisy, are a major health concern in the swine industry and are associated with important economic losses. This study aimed to investigate the antibacterial activities of essential oils against major swine respiratory pathogens with a view to developing a potential alternative to antibiotics. Their synergistic interactions with the bacteriocin nisin was also examined. Lastly, we assessed the in vitro biocompatibility of the most efficient essential oils using a pig tracheal epithelial cell line. Of the nine essential oils tested, those from cinnamon, thyme, and winter savory were the most active against Streptococcus suis, Actinobacillus pleuropneumoniae, Actinobacillus suis, Bordetella bronchiseptica, Haemophilus parasuis, and Pasteurella multocida, with minimum inhibitory concentrations and minimum bactericidal concentrations ranging from 0.01 to 0.156% (v/v). The main component found in cinnamon, thyme, and winter savory oils were cinnamaldehyde, thymol, and carvacrol, respectively. Treating pre-formed S. suis and A. pleuropneumoniae biofilms with thyme or winter savory oils significantly decreased biofilm viability. We also observed a synergistic growth inhibition of S. suis with mixtures of nisin and essential oils from thyme and winter savory. Concentrations of nisin and cinnamon, thyme and winter savory essential oils that were effective against bacterial pathogens had no effect on the viability of pig tracheal epithelial cells. The present study brought evidence that essential oils are potential antimicrobial agents against bacteria associated with porcine respiratory infections.

Characterization of pig saliva as the major natural habitat of Streptococcus suis by analyzing oral, fecal, vaginal, and environmental microbiota.

It is generally difficult to specify the sources of infection by which domestic animals may acquire pathogens. Through 16S rRNA gene amplicon sequencing, we compared the composition of microbiota in the saliva, vaginal mucus, and feces of pigs, and in swabs of feeder troughs and water dispensers collected from pig farms in Vietnam. The composition of the microbiota differed between samples in each sample group. Streptococcus, Actinobacillus, Moraxella, and Rothia were the most abundant genera and significantly discriminative in saliva samples, regardless of the plasticity and changeability of the composition of microbiota in saliva. Moreover, species assignment of the genus Streptococcus revealed that Streptococcus suis was exceptional in the salivary microbiota, due to being most abundant among the streptococcal species and sharing estimated proportions of 5.7%-9.4% of the total bacteria in saliva. Thus, pig oral microbiota showed unique characteristics in which the major species was the pig pathogen. On the other hand, ß-diversity analysis showed that the microbiota in saliva was distinct from those in the others. From the above results, pig saliva was shown to be the major natural habitat of S. suis, and is suggested to be the most probable source of S. suis infection.

Differential Effects of Breed and Nursing on Early-Life Colonic Microbiota and Immune Status as Revealed in a Cross-Fostering Piglet Model.

Nursing mother and breed can differently regulate early-life microbiota succession in pigs. However, it remains unclear whether they affect gastrointestinal microbiota and immune status, which are critical for early-life gut health. Here, an interspecific cross-fostering piglet model was employed by fostering neonatal Yorkshire and Meishan piglets to the same or another breed of sows. Jejunal and colonic microbiotas and mucosal immune parameters were analyzed at postnatal days 14 (preweaning) and 49 (postweaning). Nursing mother affected 10 genera in the colon and 3 minor genera in the jejunum. At day 14, Meishan sow-nursed piglets had lower Streptococcus suis and higher Cloacibacillus counts in the colonic digesta and larger amounts of interleukin 10 and Foxp3-positive cells in the colonic mucosa than did Yorkshire sow-nursed piglets. At day 49, nursing mother had no significant effects on cytokine expression. Breed effects were observed; Meishan piglets had lower relative abundances of Prevotella and lower gene expression of tumor necrosis factor alpha (TNF-α) than those of Yorkshire piglets at days 14 and 49. Collectively, nursing mother mainly affected preweaning colonic microbiota and immune status, while breed effects persisted after weaning. Piglets nursed by Meishan sows had different microbiota compositions and inflammatory cytokine profiles in the colon compared with those of piglets nursed by Yorkshire sows. These results highlight the different role of nursing mother and breed in affecting early gut microenvironment.IMPORTANCE Early-life gut microbiota and immune status are pivotal for postnatal growth. By using an interspecific cross-fostering piglet model, we find that change in nursing mother transiently reshapes preweaning colon microbiota and immune status, while breed shows persistent effects both pre- and postweaning. Piglets nursed by Meishan sows had lower Streptococcus suis counts and higher anti-inflammatory cytokine expression. These results highlight the significance of nursing mother in regulating early-life gut health.

In vitro/vivo Mechanism of Action of MP1102 With Low/Nonresistance Against Streptococcus suis Type 2 Strain CVCC 3928.

Streptococcosis is recognized as a leading infectious disease in the swine industry. Streptococcus suis serotype 2 is regarded as the most virulent species, which threatens human and pig health and causes serious economic losses. In this study, multiple in vitro and in vivo effects of MP1102 on multidrug resistant S. suis was studied for the first time. MP1102 exhibited significant antibacterial activity against S. suis (minimum inhibitory concentration, MIC = 0.028-0.228 µM), rapid bacteriocidal action, a longer postantibiotic effect than ceftriaxone, and a synergistic or additive effect with lincomycin, penicillin, and ceftriaxone (FICI = 0.29-0.96). No resistant mutants appeared after 30 serial passages of S. suis in the presence of MP1102. Flow cytometric analysis and electron microscopy observations showed that MP1102 destroyed S. suis cell membrane integrity and affected S. suis cell ultrastructure and membrane morphology. Specifically, a significantly wrinkled surface, intracellular content leakage, and cell lysis were noted, establishing a cyto-basis of nonresistance to this pathogen. DNA gel retardation and circular dichroism analysis indicated that MP1102 interacted with DNA by binding to DNA and changing the DNA conformation, even leading to the disappearance of the helical structure. This result further supported the mechanistic basis of nonresistance via interaction with an intracellular target, which could serve as a means of secondary injury after MP1102 is transported across the membrane. Upon treatment with 2.5-5.0 mg/kg MP1102, the survival of mice challenged with S. suis was 83.3-100%. MP1102 decreased bacterial translocation in liver, lung, spleen, and blood; inhibited the release of interleukin-1ß and tumor necrosis factor-α; and relieved the lung, liver, and spleen from acute injury induced by S. suis. These results suggest that MP1102 is a potent novel antibacterial agent for the treatment of porcine streptococcal disease.

Identification of six novel capsular polysaccharide loci (NCL) from Streptococcus suis multidrug resistant non-typeable strains and the pathogenic characteristic of strains carrying new NCLs.

Streptococcus suis is a major swine pathogen and an important zoonotic agent worldwide. At least nine serotypes can infect human so far. Although 29 serotypes (1-19, 21, 23-25, 27-31 and 1/2) strains are considered as authentic S. suis, a novel variant serotype Chz and strains carrying 20 novel capsular polysaccharide loci (NCL) have been identified recently. However, information about pathogenic and antimicrobial resistance characteristics of strains carrying NCLs is still unavailable. In this study, we identified six new NCLs (designated as NCL21-26) from 35 non-typeable S. suis strains by agglutination tests and whole genome sequencing analysis. Further analysis of the genetic context of NCL25 and NCL26 showed a mosaic structure of the capsular polysaccharide loci. NCL25 exhibited considerable similarity to that of serotypes 10 and 11, and NCL26 shared similarity to that of serotype 9 and NCL4. Antimicrobial susceptibility testing demonstrated that strains carrying NCL21-26 were all resistant to clindamycin, lincomycin, erythromycin, tilmicosin and tetracycline. Animal infection experiments showed that the virulence of NCL26 strain NJ1112 isolated from a disease pig was similar to that of S. suis serotype 2 virulent strain SC070731 in both zebrafish and mouse infection models, highlighting the necessity for surveillance of strains belonging to NCL26. We also developed a multiplex PCR assay to detect NCL21-26 strains. Our findings expand the views of genetic diversity of S. suis capsular polysaccharide loci and S. suis pathogenic characteristic.